This invention relates to a drive mechanism for windshield wiper systems on motor vehicles and includes a two-speed d.c. motor having a collector and a plurality of carbon brushes.
The windshield wiper systems of modern motor vehicles typically are driven by permanent-magnet d.c. motors, which may be driven in two different speeds of operation. In order to switch on the two different speeds of operation a total of three carbon brushes are provided, of which two rest against the collector substantially diametrically. One of these carbon brushes is connected with the ground terminal of a voltage source. The other of these two carbon brushes is connected to the positive terminal of the voltage source, when the d.c. motor is to be driven at low speed in the first speed of operation. The third carbon brush rests against the collector between the first and the second carbon brush. It is thereby set off at a particular angle relative to the carbon brush which, in the first speed of operation, may be connected with the positive terminal, which angle is in general not larger than 90.degree.. In order to driven the d.c. motor in the second speed of operation at a higher speed the third carbon brush is connected with the positive terminal of a voltage source instead of the second carbon brush.
A wiper system also is known in which the d.c. motor may be driven with three different speeds of operation. In this known system two further carbon brushes are arranged on one side of the neutral zone defined by the connecting line of the two carbon brushes connected with the voltage source in the low speed of operation, so that a total of three carbon brushes may be selectively connected with the positive terminal of the voltage source.
In a comparison with other solutions, in which different speeds are adjusted by variable series resistors or by a variable armature voltage, these known systems include considerable cost advantages. It is however true that the starting torque is smallest for the higher no-load speed selected speed of operation.
In order to circumvent this disadvantage it is known to insert a diode between the carbon brush of the lowest speed of operation and a carbon brush with a higher speed of operation, which diode provides a current flow from the lead to the carbon brush for the higher speed of operation to the carbon brush for the lowest speed of operation. However, this solution is rather expensive. Moreover it is difficult to dissipate the heat generated at the diode.
The present invention is based on the problem of developing a drive mechanism of the type described above in such a way that in the high speed of operation a starting torque of the d.c. motor is achieved which is at least substantially as great or greater than the starting torque in the low speed of operation.